This invention relates to a drive system for automobiles having two driven axles as well as a differential gearing serving as distributor gearing between these axles, with the differential gearing having a fluid friction clutch associated therewith as an optional differential lock.
The driving torque is distributed by the distributor gearing in a determined ratio to the two axles while at the same time affording the required difference between the speeds of the two axles when traveling through a curve, as the distributor gearing also acts as a differential gearing. The distribution of the driving torque is of substantial influence on the driving performance of the vehicle. As, in the known constructions, the distribution ratio can not be changed, the distribution of the driving torque achieved by the distributor gearing can only be optimal for a determined load condition.
The fluid friction clutch associated with the differential gearing consists of internal and external clutch disks which are connected firmly with corresponding disk carriers for rotation therewith and are lodged in a closed housing which is filled with a liquid of high viscosity. The two parts to be coupled of this fluid friction clutch, i.e., the inner and outer clutch disks or their disk carriers, respectively, have up to now been connected firmly with the driven or the driving members, respectively, of the differential gearing, for rotation therewith. Whenever a relative motion occurs between these driving or driven members, respectively, of the differential gearing, then a friction torque is opposed thereto by the fluid friction clutch, the size of which torque depends on the speed difference. This friction torque will then exert a locking effect on that respective driving or driven member of the differential gearing which turns faster. This represents an automatic differential lock which does not influence the transmission ratio within the differential gearing constituting simultaneously the distributor gearing. The driving torque of the two axles is influenced by the fluid friction clutch toward a rigid drive connection. Such automatic differential locks suffer from the drawback that they change the distribution of torque in an undesirable manner in the case of differences between the diameters of the wheels on the two axles due, for instance, to snow chains on the wheels of an axle, in which case the differential locks may become excessively heated, resulting in a rigid connection. Finally, the automatic lock also occurs when different speeds of the two axles result from traveling in a curve, although, particularly in this case, the effect of the differential gearing acting as a distributor gearing would be most desirable.
It is also known from the German Offenlegungsschrift No. 32 12 495 to associate with a differential gearing a lock formed by a simple friction or disk clutch, and using in this case, as controlling characteristic magnitude, for the coupling starter of this clutch, either the engine torque, the negative pressure in the suction tube, the position of the accelerator pedal, the steering power, the steering angle, the braking force, the axial load distribution, the wheel slippage, the traveling speed and/or the position of the gear shift lever of the gearing. In doing so, it may be sufficient to only engage or disengage the gearing lock, while only influencing the distribution of the driving torque to the two driven axles in a limited manner, quite apart from the fact that it is not possible to effect a distribution of the driving torque independently of the prevailing load on the respective axle.